This invention relates generally to micro-electro-mechanical systems (MEMS), and more particularly to MEMS apparatus and methods for making MEMS apparatus, such as mirrors, as by a combination of bulk and surface micromachining techniques.
MEMS apparatus, such as mirrors, have utility in a variety of optical applications, including high-speed scanning and optical switching. In such applications, it is essential for MEMS mirrors to have flat optical surfaces, large rotational range, and robust performance.
Many of these optical applications, e.g., optical networking applications, further require that MEMS mirrors be configured in a closely packed array. It is desirable in such applications to maximize the “optical fill factor” of the array, e.g., by making the optical surface area of each constituent mirror as large as possible relative to its supporting base area. In known MEMS mirrors, the hinges and associated structure that are necessary to permit the mirrors to be actuated, e.g., rotated, to reflect the focused beam to a desired location, limit the permissible size of the mirror surface. This results in a sub-optimum optical fill factor and, in optical networking applications, a sub-optimum passband. This is particularly true for mirrors which are biaxially movable, since two orthogonal sets of hinges and an associated gimbal or equivalent structure are required. This necessitates a greater space between adjacent minors to accommodate the hinges and associated structure.
MEMS mirrors are conventionally made by either bulk or surface silicon micromachining techniques. Bulk micromachining, which typically produces single-crystal silicon mirrors, is known to have a number of advantages over surface micromachining, which typically produces polysilicon (thin-film) mirrors. For example, single-crystal silicon mirrors produced by bulk micromachining techniques are generally thicker and larger mirrors with smoother surfaces and less intrinsic stress than polysilicon mirrors. Low intrinsic stress and sizeable thickness result in flat mirrors, while smooth surfaces reduce undesired light scattering. An advantage inherent to surface micromachining techniques is that the mirror suspension, e.g., one or more thin-film hinges, can be better defined and therefore made smaller. This allows the MEMS mirror thus produced to have a large rotational range at moderate drive voltages.
U.S. Pat. No. 6,028,689 to Michalicek et al. (“Michalicek et al.”) discloses a movable micromirror assembly driven by an electrostatic mechanism. The assembly includes a mirror supported by a plurality of flexure arms situated under the mirror. The flexure arms are in turn mounted on a support post. Because the assembly disclosed by Michalicek et al. is fabricated entirely by way of surface micromachining techniques, the resulting “micromirror” is of the polysilicon (thin-film) type, and is thus subject to the aforementioned disadvantages.
Published International Patent Application No. WO 01/94253 of Chong et al. discloses a MEMS mirror device having a bulk silicon mirror attached to a frame by thin-film hinges. A notable shortcoming of this system is evident in that the thin-film hinges extend from the reflective surface side of the mirror to the frame, hence restricting (or obstructing) the amount of surface area available for optical beam manipulation. This shortcoming further results in a lower optical fill factor in an array of such MEMS devices.
Tuantranont et al. in “Bulk-Etched Micromachined and Flip-Chip Integrated Micromirror Array for Infrared Applications,” 2000 IEEE/LEOS International Conference on Optical MEMS, 21024, Kauai, Hawaii (August 2000) disclose an array of deflectable mirrors fabricated by a surface micromachining polysilicon (or “MUMPS”) process. An array of polysilicon mirror plates is bonded to another array of thermal bimorph actuators by gold posts using the “flip-chip transfer technique”, resulting in trampoline-type polysilicon plates each suspended at its corners by thermal bimorph actuators. In addition to the mirror plates being made of polysilicon (or thin-film), another drawback of the mirror array is the lack of a monolithic structure, which makes the array susceptible to misalignment and other extraneous undesirable effects.
In view of the foregoing, there is a need in the art to provide MEMS apparatus, such as mirrors, that overcome the limitations of prior devices and which have a simple and robust construction.